1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus for recording/reproducing data using an optical recording medium such as an optical card.
2. Description of the Related Art
In recent years, a technique of data processing has been developed, and media for recording a large amount of data have been required. As one of the media, an optical recording medium has received a great deal of attention. An optical card is known as one of the optical recording media. As an apparatus for recording data onto the optical card and reproducing data of the optical card, an optical card recording/reproducing apparatus is practically used.
In the optical card, a laser beam is radiated through a lens on a data recording layer having a high reflectance and arranged on a substrate having the same shape as that of a credit card to form pits (holes) having a low reflectance on the recording layer by a thermally irreversible change, thereby writing data on the recording layer. The optical card has a recording capacity several or ten thousand times that of a conventionally used magnetic card. Although data cannot be rewritten in the optical card as in an optical disk, the recording capacity of the optical card is very large, i.e., 1 to 2 MG. Therefore, optical cards can be used in a variety of applications such as bankbooks, portable maps, prepaid cards used in shopping or the like. In addition, since the optical card has the characteristic feature of inhibiting data rewrite, the optical card can be used in applications such as individual health management cards which does not allow illegal data updating.
FIG. 1 is a plan view showing an optical card. A data recording portion 2 is formed at the central portion of a card main body 1, and ID portions 3a and 3b in which identification data such as track addresses are recorded are formed on both ends of the data recording portion 2.
The data recording portion 2 has, as shown in FIG. 2, a plurality of track guides 201, having a low reflectance, for guiding a light beam such as a laser beam in the track directions, and tracks 202 having a high reflectance and formed between the guide tracks 201. Data pits 203 having a low reflectance and representing recorded data are formed along the tracks 202. Although the tracks 202 are formed to extend over the entire length of the card main body 1, the end portions of the tracks are easily damaged or contaminated and have poor reliability. Further, in order to sufficiently stabilize a relative moving speed between the optical card and an optical head in the track directions, the ID portions 3a and 3b are formed at positions inward from the ends of the card at predetermined distances (e.g., 4 mm). The data recording portion 2 is defined between the ID portions 3a and 3b. Since data are read from both the directions of the optical card while the optical card is reciprocally conveyed, the ID portions 3a and 3b are formed so that track addresses can be read from both directions. Therefore, in FIG. 1, when a light beam is moved from the left to the right along the tracks, the left ID portion 3a is read; when a light beam is moved from the right to the left, the right ID portion 3b is read, thereby identifying a tracking address. Thus, the ID data such as the tracking address can be read out regardless of a scanning direction before data is read out.
The optical system of an optical head for reproducing data of the optical card is shown in FIG. 3. A laser beam emitted from a light-emitting element 4 such as a laser diode is collimated into a parallel beam by a collimator lens 5, diffracted by a diffraction grating 6, and focused on the optical card 1 through an objective lens 7. The focused light is reflected by the optical card 1, and is incident on a detector 10 through a mirror 8 and a detection system lens 9.
The focused light beam on the optical card 1, as shown in FIG. 2, is constituted by a 0th-order diffracted beam 601 called a main beam and two 1st-order diffracted beams 602 and 603 called sub-beams, all of which are diffracted by the diffraction grating 6. The main beam 601 is used to reproduce the data pits 203 or to generate a focus error signal for focusing control, and each of the sub-beams 602 and 603 is radiated half on a corresponding one of the track guides 201 and is used to generate a tracking error signal.
The light beam reflected by the optical card 1 and incident on the detector 10, as shown in FIG. 4, is constituted by three beams. Light beams 101, 102, and 103 shown in FIG. 4 correspond to light beams 601, 602, and 603 shown in FIG. 2, respectively. In the detector 10, an optical system is constituted such that the light beam 101 is radiated on the dividing line of detection regions 101a and 101b obtained by dividing a square detection region, the light beam 102 is radiated on the center of a detection region 102a, and the light beam 103 is radiated on the center of a detection region 103a.
In addition, the optical system is constituted such that the light beam 101 is moved in the direction perpendicular to the dividing line of the detection regions 101a and 101b when the beam on the optical card is defocused. Therefore, when a difference between amounts of light incident on the detection regions 101a and 101b is calculated, a focus error signal representing an error of the in-focus position can be obtained. When the objective lens 7 is driven by a driving means 11 according to the focus error signal to be brought close to or separated from the card 1, focusing control is performed such that the light beam is kept in an in-focus state on the card.
When the beams 602 and 603 shown in FIG. 2 are moved in the direction perpendicular to the tracks 202, the overlapping areas of the beams 602 and 603 on the track guides 201 are changed. Therefore, tracking error signals representing errors of the beams 602 and 603 from the centers of the track guides 201 can be obtained by calculating an output difference between the detection regions 102a and 103a. The objective lens 7 is moved by the driving means 11 in the direction perpendicular to the track guides 201 according to the tracking error signal, so that tracking control is performed to keep the beam 601 at the center of each of the tracks 202.
FIG. 5 is a view for explaining an access method of an arbitrary track, which method is employed in the optical data recording/reproducing apparatus arranged as described above.
In FIG. 5, as in FIGS. 2 and 4, reference numerals 201 denote track guides for guiding the light beam in the track directions; 202, tracks formed between the track guides 201; 601, a main beam for reproducing data pits formed along the tracks 202 and for generating focus error signals; and 602 and 603, sub-beams, each formed to half overlap a corresponding one of the track guides 201, for generating tracking error signals. In FIG. 5, both the ends of the tracks 202 represent both the ends of the card, arrows a, b, c, and d indicate the moving direction of the beam on the card. In general, the beam is moved on the card in a direction a or c perpendicular to the tracks by moving an optical head itself or an objective lens, and the beams on the card are moved in a direction b or d parallel to the tracks by moving the card with respect to the optical head.
Data reproduction of a given track is designated by an external apparatus (not shown) such as a host computer. This given track designated by a reproduction request is referred to as a target track address. In the example of FIG. 5, the target address is represented by AD1. A light beam is moved in the direction of an arrow a by a difference between a track address AD at which the main beam 601 is currently located and the target track address AD1.
Since the distance in the direction of the arrow a to move the beam from the current address to the target address is generally long, the light beam is moved by moving the entire optical head. Access performed such that the entire optical head is moved is called "coarse access". In the coarse access, the position of the moved beam is set within an error range around the target track, and the light beam cannot always reach the target track by performing coarse access once because of the following reasons. A scale used in the coarse access has an insufficient accuracy, the objective lens is vibrated, and the apparatus itself is vibrated, or the like.
In FIG. 5, it is assumed that the beams 601, 602, and 603 are moved three tracks before the target track address AD1 (i.e., a track address AD0) due to a coarse access error. Thereafter, the card is moved in the direction parallel to the tracks, the beams 601, 602, and 603 are relatively moved in the direction of an arrow b, so that the above-described ID portion 3a is reproduced. When it is determined by the readout data from the ID portion 3a that the current track address is AD0, the beams 601, 602, and 603 are moved again by a distance corresponding to the difference between the current track address AD0 and the target track address AD1. This moving distance in this case is shorter (about 1 to 8 tracks at most) than the distance in the direction of the arrow a, and the beams are moved by changing the irradiation positions of the beams by shifting the objective lens 7 track by track. This operation is called a track-jump operation. Short-distance moving performed by repeating the track-jump operation is called "fine access". Since the fine access has no error factors unlike the coarse access, accurate moving can be performed. Note that, when there is a reproduction request, and the difference between a current track and a target track is small, the fine access is first performed without performing the coarse access.
In FIG. 5, the track-jump operation is performed three times as indicated by arrows c1, c2, and c3 to cause the beam 601 to reach the track 202 of the target track address AD1. Thereafter, the card is moved in the direction parallel to the tracks, and the beams 601, 602, and 603 are relatively moved in the direction of an arrow d, so that the ID portion 3b is reproduced. When it is confirmed by the read data from the ID portion 3b that the track address of the current track is AD1, data is reproduced from the data recording portion 2.
Data is recorded in a predetermined direction, but the data is reproduced in two forward and backward directions, so that a scanning direction may be reversed between write access and read access. Therefore, reproduced data is temporarily written in a buffer memory, and a data string is inverted in a direction of time axis in accordance with the reproduction direction, so that the data can be correctly read out in two forward and backward directions.
When a moving distance between the current track address and the target track address is long as described above, the target track cannot easily be accessed by performing coarse access once, and an operation constituted by coarse access and fine access is performed. Therefore, a total of at least two track scannings must be respectively performed after the coarse access and after the fine access. As a result, a long time is required for a reproduction operation, and the reproduction operation cannot be performed at very high speed.
The drawback of the above access method is also applicable to the recording operation.